1. Field of the Invention
The invention relates to a process for the cold molding of particulate materials, in particular of fiber materials, using, as binders, non-crosslinkable, thermoplastic polymers, in the form of polymer powder or in the form of an aqueous polymer dispersion.
2. Background Art
Composite materials based on fiber materials are used in the automotive industry, aircraft industry, and construction industry. The sheets have to be stiff and sound-absorbent, and have to have adequate heat resistance. There are various production processes for these composite materials. An example of an application for the automotive industry is the roofliner. To produce roofliners, the desired properties are achieved via the combination of polyester fibers and bicomponent polyester fibers (bico fibers) with glass fiber mats, the glass fiber mats being used to increase stiffness and heat resistance. The bico fibers have a core material with high melting point and a shell with low melting point. In the cold molding process, polyester nonwovens and glass fiber mats are heated at a temperature above the melting point of the fiber shell material and then press-molded at low temperature to give a molding. The temperatures in the heating phase are from 160 to 240xc2x0 C., depending on melting point, and the molding temperature is well below the melting point of the fiber shell, at from about 20 to 60xc2x0 C. In the press, the fibers become bonded during cooling under pressure. This procedure is termed cold molding.
Since the processing of glass fiber leads to skin irritation and poses problems during recycling, novel solutions are being sought which do not require such fibers. U.S. Pat. No. 6,214,456 describes the production of a roofliner using bico fiber, polyurethane foam being used for reinforcement instead of glass fiber. A disadvantage is the inadequate sound absorption of the resultant composite materials. In U.S. Pat. No. 6,156,682, the composite material is reinforced by using thermally crosslinkable (thermosetting) polymers, without glass fiber. U.S. Pat. No. 5,888,616 discloses the disadvantages of the use of thermally crosslinkable polymers: the materials bonded therewith cannot later be recycled. The latter publication therefore describes the production of recyclable materials using glass fiber, polyester fiber, and polyester bico fiber. The polyester fibers have a variety of titers and melting points.
An object of the present invention is to provide composite materials which, even without glass fiber content, have high heat resistance and are recyclable. These and other objects are met through the use of specific polymer powders or aqueous dispersions thereof as cold molding binders.
The invention thus provides a process for the cold molding of particulate materials, where the binder and the material to be bound are first preheated to a temperature of from 160 to 240xc2x0 C., and then, preferably within a period of from 1 to 20 sec, at a temperature less than 80xc2x0 C., and at a pressure of from 1 to 120 bar, are press-molded to give a molding, wherein as a binder, a polymer in the form of a polymer powder or in the form of an aqueous polymer dispersion thereof is employed, the glass transition temperature Tg of the polymer being greater than 80xc2x0 C. and its complex viscosity being less than 15,000 Paxc2x7s at 190xc2x0 C., made from one or more comonomer units from the group consisting of vinyl esters of unbranched or branched (xe2x80x9coptionally branchedxe2x80x9d) alkylcarboxylic acids having from 1 to 18 carbon atoms, (meth)acrylic esters of branched or unbranched (xe2x80x9coptionally branchedxe2x80x9d) alcohols having from 1 to 15 carbon atoms, dienes, olefins, vinylaromatics, and vinyl halides.
Suitable vinyl esters are vinyl esters of optionally branched carboxylic acids having from 1 to 18 carbon atoms. Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate, and vinyl esters of xcex1-branched monocarboxylic acids having from 5 to 11 carbon atoms, examples being VeoVa9(copyright) or VeoVa10(copyright) (trade names of Shell). Vinyl acetate is particularly preferred.
Suitable monomers from the (meth)acrylic ester group are esters of optionally branched alcohols having from 1 to 15 carbon atoms. Preferred (meth)acrylic esters are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, 2-ethylhexyl acrylate, and norbornyl acrylate. Particular preference is given to methyl acrylate, methyl methacrylate, n-butyl acrylate, 2-ethylhexyl acrylate, and norbornyl acrylate.
Suitable dienes are 1,3-butadiene and isoprene. Examples of copolymerizable olefins are ethene and propene. Vinylaromatics which may be copolymerized include styrene and vinyltoluene. From the group of the vinyl halides, vinyl chloride is usually used.
In one preferred embodiment, the polymer also contains from 0.1 to 50% by weight, based on the total weight of the polymer, of one or more comonomer units from the group consisting of carboxy-functional and hydroxy-functional monomers.
Suitable carboxy-functional comonomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, crotonic acid, itaconic acid, fumaric acid, and maleic acid. The carboxy function may also be introduced into the copolymer by copolymerizing maleic anhydride. Suitable hydroxy-functional comonomers are hydroxyalkyl acrylates and hydroxyalkyl methacrylates having a C1-C8-alkyl radical, preferably hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate.
Particular preference is given to carboxy-functional comonomer units, those most preferred being those derived from acrylic acid, from methacrylic acid, from crotonic acid, from itaconic acid, from fumaric acid, from maleic acid, or from maleic anhydride. The proportion of functional units is particularly preferably from 0.1 to 25% by weight, most preferably from 3 to 20% by weight, based in each case on the total weight of the copolymer.
Preference is given to the polymers mentioned below, which optionally also contain the functional-group-containing units as described immediately above: vinyl ester-ethylene copolymers such as vinyl acetate-ethylene copolymers; vinyl ester-ethylene-vinyl chloride copolymers, where the vinyl ester is preferably vinyl acetate and/or vinyl propionate and/or one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, or vinyl esters of an alpha-branched carboxylic acid having from 5 to 11 carbon atoms, in particular vinyl versatate (VeoVa9(copyright), VeoVa10(copyright)); vinyl acetate copolymers with one or more copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate, vinyl 2-ethylhexanoate, or vinyl esters of an alpha-branched carboxylic acid having from 5 to 11 carbon atoms, in particular vinyl versatate (VeoVa9(copyright), VeoVa10(copyright)), optionally containing ethylene as well; vinyl ester-acrylic ester copolymers, in particular those of vinyl acetate and butyl acrylate and/or 2-ethylhexyl acrylate, optionally also containing ethylene; vinyl ester-acrylic ester copolymers of vinyl acetate and/or vinyl laurate, and/or vinyl versatate as the vinyl esters, and butyl acrylate or 2-ethylhexyl acrylate as the acrylic esters, further optionally containing ethylene.
Particular preference is given to (meth)acrylic ester polymers and to styrene polymers, for example copolymers of n-butyl acrylate and/or 2-ethylhexyl acrylate; copolymers of methyl methacrylate with butyl acrylate and/or 2-ethylhexyl acrylate, and/or 1,3-butadiene; styrene-1,3-butadiene copolymers and styrene (meth)acrylic ester copolymers such as styrene-butyl acrylate, styrene-methyl methacrylate-butyl acrylate, or stryene-2-ethylhexyl acrylate, where the butyl acrylate used may comprise n-, iso-, or tert-butyl acrylate.
The greatest preference is given to carboxy-functional styrene-n-butyl acrylate copolymers, carboxy-functional methyl methacrylate-n-butyl acrylate copolymers, and carboxy-functional styrene-methyl methacrylate-n-butyl acrylate copolymers, the carboxy groups being introduced by way of acrylic acid units and/or methacrylic acid units.
The selection of the monomers and/or their proportions by weight is such that the polymers have a glass transition temperature Tg greater than 80xc2x0 C. The glass transition temperature Tg of the polymers may be determined in a known manner by differential scanning calorimetry (DSC). The Fox equation may also be used for an approximate preliminary calculation of the Tg. According to T. G. Fox, Bull. Am. Physics Soc. 1, 3, page 123 (1956): 1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn, where xn is the fraction by weight (% by weight/100) of the monomer n, and Tgn is the glass transition temperature in Kelvin of the homopolymer of the monomer n. Tg values for homopolymers are listed in Polymer Handbook 2nd Edition, J. Wiley and Sons, New York (1975).
The polymers may be prepared by known free-radical-initiated polymerization processes, for example by solution polymerization, aqueous suspension polymerization, or aqueous emulsion polymerization. Emulsion polymerization is preferred. Any of the familiar drying processes may be used to dry the solutions or dispersions: spray drying, drum drying, freeze drying, belt drying or coagulation followed by fluidized-bed drying. The polymers are preferably prepared using spray-drying or drum-drying processes. During the drying process, it is also possible, where appropriate, to add additives such as flame retardants, plasticizers, fillers, or complexers.
Complex viscosity is understood to mean dynamic viscosity. P. Carreau, Rheology of Polymeric Systems, Carl Hanser Verlag 1997. The complex viscosity of the polymer may be adjusted in a known manner via the molecular weight and/or the amount of functional comonomer units. Preference is given to a complex viscosity at 190xc2x0 C. of less than 8000 mPas.
In cold molding, the polymer, in the form of polymer powder or in the form of aqueous polymer dispersion, preferably in the form of polymer powder, is used to produce moldings from particulate materials such as fiber materials, from particulates composed of mineral materials, from plastics, or from natural substances such as wood chips, cork particles, and glass particles, or from combinations of these materials. The preferred application is the application as a binder for fiber materials. Natural or synthetic raw fiber materials are suitable. Examples of these are synthetic fibers based on fiber-forming polymers, examples being viscose fiber, polyester fiber such as chopped polyester fiber, polyamide fiber, polypropylene fiber, polyethylene fiber, polyethylene terephthalate (PET) fiber, bicomponent polyester fibers (bico fibers), e.g. high-crystallinity bicomponent polyester fibers, and fiber mixtures made from these fibers. Mineral fibers are also suitable, examples being glass fibers, ceramic fibers, and carbon fibers. Examples of natural fiber materials are wood fibers, cellulose fibers, wool fibers, cotton fibers, jute fibers, flax fibers, hemp fibers, coir fibers, ramie fibers, and sisal fibers. The fibers may also be in the form of wovens, in the form of yarns, or in the form of nonwovens such as nonwoven scrims or formed-loop knits. These nonwovens may, where appropriate, have been mechanically preconsolidated, for example by needling.
The amount of the polymer generally used in cold molding is from 5 to 90% by weight, preferably from 20 to 70% by weight, based on the material to be bound. The amount of binder depends on the desired stiffness of the press-molded molding.
One procedure for producing the moldings is to mix the fibers with the polymer powder or with the aqueous polymer dispersion, laying up the fiber/polymer mixture by conventional nonwoven technology methods, optionally after carding of the fiber/polymer mixture and/or needling, followed by preheating and cold molding in the abovementioned pressure and temperature ranges. Another method of binding the fiber is to scatter or spray the binder composition into a woven fabric or nonwoven scrim, or into a fiber bed, where appropriate after carding the fiber/polymer mixture and/or needling, and then to preheat and cold mold in the abovementioned pressure and temperature ranges.
The process for the cold molding of particulate materials is preferably used to produce moldings for interior automotive trim, most preferably for producing automotive roofliners. The use of binders based on non-crosslinkable thermoplastic polymers permits the production of automotive roofliners with adequate stiffness and with heat resistance at 85xc2x0 C. from polyester fibers such as PET, without any use of glass fibers. Due to the thermoplastic character of the binder, these moldings differ from thermoset binders in being recyclable.